1. Field of the Invention
The present invention generally relates to electrophotography, and more particularly, to a color electrophotographic apparatus which may be applied to a color copying apparatus, color printer or the like.
Generally, for forming a color image by electrophotography, it has been a practice to form a color image by overlapping toner images of yellow, magenta, cyan and black respectively on a transfer material. As the method for effecting the overlapping of such toner images over the transfer material, there have been generally employed a transfer drum system in which the transfer material wound on a transfer drum is rotated and repeatedly brought into the same image forming position, whereat toner images of respective colors successively formed are overlapped for transfer, and a continuous transfer system wherein a plurality of image forming sections are disposed side by side, and the transfer material transported by a transport belt or the like is caused to pass through the transfer position of each image forming section, thereby to successively transfer and overlap the toner image of the respective colors to obtain the colored image.
One example of color image forming apparatuses employing the former transfer drum system is disclosed, for example, in Japanese Patent Laid-Open Publication Tokkaihei No. 1-252982, which will be referred to as a prior art (1) hereinafter.
Referring to FIG. 16, general construction and function of the color image forming apparatus of the prior art (1) will be briefly explained.
In FIG. 16, the conventional color image forming apparatus includes a housing H, and a photosensitive drum 201 having a photosensitive material layer provided on its outer peripheral surface, and rotatably provided at generally a central portion of the housing H. Around the photosensitive drum 201, there are sequentially disposed various processing devices such as a corona charger 202, a developing section 203, a transfer drum 204, and a cleaner 205, etc. The developing section 203 further includes a Y developing unit 206 for forming a yellow toner image, an M developing unit 207 for forming a magenta toner image, a C developing unit 208 for forming a cyan toner image, and a K developing unit 209 for forming a black toner image, which are arranged to rotate so that the respective developing units may be successively brought into a position confronting the photoreceptor drum 201 at which the developing may be effected During functioning, the transfer drum 204 and the photosensitive drum 201 confronting each other are rotated in directions indicated by arrows at constant speeds.
In the first place, upon starting of the functioning, the photosensitive drum 201 is rotated in the direction of the arrow, and the photosensitive surface thereof is uniformly charged by the corona charger 202. Thereafter, a laser beam BM modulated by a signal for forming a first yellow image is projected as shown by a chain line onto the surface of the photosensitive drum 201 so as to form an electrostatic latent image thereon, which is further developed by the developing unit 206 for yellow initially confronting said photosensitive drum 201, and thus, the yellow toner image is formed. By the time when the yellow toner image thus formed arrives at the position confronting the transfer drum 204, a copy paper sheet as a transfer material (not particularly shown) fed from a paper feeding section 211 has already been wound on the outer periphery of the transfer drum 204, with the leading edge of the copy paper sheet being grasped by a claw portion 212, and timing is so arranged that the yellow toner image on the photosensitive drum 201 confronts and meets a predetermined position of the copy paper sheet, thereby to from the toner image.
After the yellow toner image on the photosensitive drum 201 has been transferred onto the copy paper sheet by the action of the transfer charger 213, the photosensitive surface of the drum 201 is cleaned by the cleaner 205 to prepare for the image formation in the subsequent color. Sequentially, the magenta, cyan, and black toner images are formed in the similar manner, and in such case, the developing section 203 causes each of the respective developing units used according to the colors, to confront the photosensitive drum 201 so as to be ready for the development. The transfer drum 204 has a diameter sufficient to wind a copy paper sheet of the maximum length therearound, and also, to provide enough time to allow exchange of the developing units between images of the respective colors.
The projection of the laser beam BM for the image formation of the respective colors is so timed that the toner image of each color on the photosensitive drum 201 and the toner image already transferred onto the copy paper sheet on the transfer drum 204 confront each other, with positional coincidence as the drums 201 and 204 rotate. Thus, the toner images of four colors are transferred in the overlapped state onto the copy paper sheet on the transfer drum 204, and thus, the color image is formed on the copy paper sheet. After the toner images of all colors have been transferred, the copy paper sheet is separated from the transfer drum 204 by a separating claw 214, and fed through a transport section 215, to a fixing device 216 so as to be fixed with the above toner image, and then, discharged out of the apparatus.
Meanwhile, another example of color electrophotographic apparatuses based on the latter continuous transfer system is disclosed, for example, in Japanese Patent Laid-Open Publication Tokkaihei No. 2-16580, which will be referred to as a prior art (2) hereinafter.
FIG. 17 shows a printer portion which is an image forming section of the copying apparatus disclosed in the above Tokkaihei No. 2-16580 as the prior art (2).
In the arrangement of FIG. 17, four image forming sections 222BK, 222C, 222M and 222Y respectively having photosensitive drums 223BK, 223C, 223M and 223Y, and exposure devices 221BK, 221C, 221M and 221Y, etc. for the image formation in four colors are aligned side by side, and a copy paper sheet transported by a transport belt 227 passes through respective transfer sections for the photosensitive drums so as to be overlapped with the toner images. Digital signals sent from an image reading section (not shown) are applied to the printer section, and fed to the laser exposure devices of respective color signals, i.e., 221BK for black, 221C for cyan, 221M for magenta, and 221Y for yellow. Since each of the image forming sections 222 has similar construction, only the image forming section 222C for cyan will be described hereinbelow, and like parts in other image forming sections being designated by like reference numerals with suffixes of respective colors for brevity of explanation.
The image forming section 222C has the photosensitive drum 223C besides the exposure device 221C, and around said photosensitive drum 223C, there are sequentially disposed a corona charger 224C, a developing device 225C, a transfer charger 226C, etc. in the known manner as in a conventional electrophotographic copying apparatus. The photosensitive surface of the photosensitive drum 223C uniformly charged by the corona charger 224C is formed with an electrostatic latent image for the cyan image through exposure by the exposure device 221C, and the latent image is developed into a visible toner image by the developing device 225C. The copy paper sheet (not particularly shown) transported by a transfer belt 227 is successively fed to the photosensitive drums 223BK, 223C, 223M and 223 Y respectively formed with the visible toner images of the respective colors, and is transferred with the toner images by the action of the transfer charger 226, whereby a full color image is obtained on the transfer sheet.
Furthermore, as another method for forming a color image by overlapping toner images of different colors on a transfer material, U.S. Pat. No. 4,905,084 discloses a method for overlapping toner images by repeating transfer through reciprocating displacement of the transfer material with respect to a photosensitive material.
In the conventional arrangements as described above, by the transfer drum system of the prior art (1), the transfer drum is employed for positioning and overlapping toner images of different colors. By rotating the transfer drum at the same speed with respect to the photosensitive drum, and further, by coinciding timing thereof with respect to the leading edge of the image, mutual positions of the toner images in the respective colors are registered in the case where the color image is to be formed.
In the known arrangement as described above, however, the copy paper sheet must be wound onto the transfer drum, requiring the diameter of the transfer drum to be larger than a predetermined size, with the construction thereof being very complicated, and thus, the apparatus tends to be on a large scale. Moreover, tough or hard paper such as a post card, cardboard or the like can not be employed, since such paper is difficult to be wound onto the transfer drum.
On the other hand, in the continuous transfer system as in the prior art (2) in FIG. 17, since the image forming positions corresponding to the number of colors are provided and the copy paper sheet is only required to successively pass therethrough, the transfer drums are not necessary. However, in this case, due to the fact that the image forming positions equal to the number of colors are required, the exposure means for exposing images onto the photosensitive drum is necessary by the number of colors, and thus,. cost of the apparatus is undesirably increased. Moreover, in the above arrangement, as is also disclosed in Japanese Patent Laid-Open Publication Tokkaihei No. 1-250970, high accuracy is required for the positioning of the respective colors of the exposure devices, and thus, particular structure and complicated arrangements are necessary for the image exposure system. Further, since the image forming positions are present separately at a plurality of places, positioning of respective colors on the transfer material also becomes difficult, and together with the difficulty in the positioning for the exposure devices as described earlier, color shift or deviation tends to be large.
Additionally, in the arrangement disclosed in U.S. Pat. No. 4,905,048 referred to earlier in which the transfer material is reciprocatingly displaced, since the intersection of the developing section with the belt is prevented so that toner images in various colors may be formed on one photosensitive member, there have been such problems that (1) the size of the photosensitive member tends to be large, (2) a spacing/contacting mechanism between the photosensitive member and belt is required in addition to the change-over mechanism of the developing units for respective colors, thus complicating the construction and control therefor, and (3) maintenance work for the replacement, etc. of such mechanisms also becomes complicated.
2. Description of the Prior Art